The position of an elevator car in an elevator system is best determined directly using a sensor or encoder which is directly linked to the elevator car. This is however expensive since it is necessary to provide a supplementary arrangement for measuring the position of the elevator car.
Another approach is, as described exemplarily in the patent document WO 2012/032020, to use an incremental rotary encoder connected with a pulley of the elevator system to determine the position of the car.
This method is not particularly satisfying, since the traction and suspension means of the elevator may stretch over time and due to the loading of the car, leading to a drift of the car position and consequently to stopping inaccuracy of the car. Slipping of the traction means may also cause inaccuracy in the determination of the position of the car.
In addition, when a car is reaching stopping at a floor, the car may be too fast or too slow. This may impede the rotary encoder of the drive or the control unit of the elevator system to properly control the elevator system and stop the elevator car such as the flag reading sensor lies in the middle of the flag marking, as suggested by the patent document WO 2012/032020. This also leads to a stopping inaccuracy of the car such that a step is present between the car floor and the building floor, said step being e.g. potentially dangerous for passengers of the elevator car.